The present invention relates to surfaces which can alter their wetting properties when subjected to an external stimulus. More particularly, the present invention relates to photoresponsive surfaces, and methods for altering their wettability, articles comprising such surfaces and methods for preparing said surfaces.
Wetting plays a decisive role in the success or failure of many industrial and natural Photographic film production, pigment dispersion, mineral flotation, the movement of water in soils, printing, optical filters and aspects of gene therapy are all controlled in large measure by wetting and dewetting processes. The liquid phase involved is most commonly, but not exclusively, water.
The wetting of a surface is characterized by the Young equation
xcex3SV=xcex3SL+xcex3LV cos xcex8xe2x80x83xe2x80x83(I) 
which describes the balance between the interfacial tensions (xcex3) which exist at the three-phase line of contact between solid (S), liquid (L) and vapour (V). A change in the wetting of a surface by a liquid is reflected by the contact angle (xcex8) which is measured through the denser phase as the angle that the tangent to the liquid-vapour interface makes with the solid surface at the contact line.
The change in wettability and in the contact angle is predicted by                               cos          ⁢                      xe2x80x83                    ⁢                      θ            ⁡                          (                              p                ⁢                                  xe2x80x83                                ⁢                H                            )                                      =                              cos            ⁢                          xe2x80x83                        ⁢                          θ              ⁡                              (                                  p                  ⁢                                      xe2x80x83                                    ⁢                                      H                    pzc                                                  )                                              -                                    Δ              ⁢                              xe2x80x83                            ⁢                                                F                  dl                                ⁡                                  (                                      p                    ⁢                                          xe2x80x83                                        ⁢                    H                                    )                                                                    γ              Iv                                                  1      
where xcex8 is the contact angle at the solid-liquid-vapour interface.
xcex3Iv is the liquid-vapour surface tension, pHpzc is the pH where the surface bears zero charge and xcex94Fdl is the free energy of double layer formation. Correspondingly, the free energy of formation of a single double layer is given by                               Δ          ⁢                      xe2x80x83                    ⁢                      F            dl                          =                  -                                    ∫              0                              ψ                0                                      ⁢                                          σ                0                            ⁢                              xe2x80x83                            ⁢                              ⅆ                ψ                                                                2      
where xcexa80 is the electrical potential of the solid-liquid interface and "sgr" is the surface charge. This equation is valid for Nernstian surfaces i.e. for those for which xcexa80(pX) (where X is the potential determining ion) obeys the Nernst equation. For non-Nerstian surfaces, configurational contributions can be included in equation 2. The xcex94Fdl contribution in Equation 2 can be readily calculated from electrical double layer theory. For Nernstian surfaces this is achieved by calculating xcex80(xcexa80) from the Polsson-Boltzmann equation and performing the integration in Equation 2, In this case, for a flat diffuse double layer,                               Δ          ⁢                      xe2x80x83                    ⁢                      F            dl                          =                              -                                          8                ⁢                                  n                  0                                ⁢                kT                            κ                                ⁢                      {                          cos              ⁢                              xe2x80x83                            ⁢              h              ⁢                                                zeψ                  0                                                  2                  ⁢                  kT                                                      }                                      3      
where n0 is the concentration of the symmetric z:z electrolyte, k is the Boltzmann constant, xcexa the reciprocal double layer thickness and e the elementary charge. The relationship between surface charge, pH and the influence of ionic strength is thus complete. By way of illustration only, when a molecular surface ionizes as the pH increases above its pKa, say, by one pH unit, where H+sq is the potential determining ion, the increased surface charge causes the contact angle to decrease and the surface becomes more wettable with respect to the wetting phase. Thus, by changing the surface charge or pKa of a surface, its wetting properties can conceivably be altered.
It has now been found that certain molecules attached as a thin layer or film to the surface of a substrate, impart a photoresponsitivity to the substrate surface such that the wettability of the surface changes when it is irradiated with light of an appropriate wavelength.
Accordingly, in a first aspect, the present invention provides an article having a photoresponsive surface, said article comprising a substrate having photoionisable moieties capable of undergoing dimerization attached to at least a portion of a surface thereof, the proximity of said moieties to one another being such that irradiation with light of an appropriate wavelength results in dimerization of at least a portion of said moieties thereby altering the wettability of the surface.
In another aspect, the present invention also provides a method for preparing a substrate having a photoresponsive surface which method comprises attaching to at least a portion of the substrate surface, photoionisable moieties capable of undergoing dimerization wherein the proximity of said moieties to one another on the substrate surface is such that irradiation of said moieties with light of an appropriate wavelength results in dimerization of at least a portion of said moieties, thereby altering the wettability of the surface.
In yet a further aspect, the invention provides a method for altering the wettability of a surface of a substrate, said substrate having attached to at least a portion of said surface photoionisable moieties capable of undergoing dimerization, wherein the proximity of said moieties to one another on said surface is such that dimerization can occur, said method comprising irradiating said surface with light at an appropriate wavelength sufficient to dimerize at least a portion of the photoionisable moieties.
The present invention is based on the finding that certain molecules, when irradiated by light of an appropriate wavelength, can covalently couple to give a dimeric form which has a different pKa value to that of the individual molecule. When such molecules (i.e. the monomer) are attached to a substrate surface, in a way that they can dimerize, this can provide a photoresponsive surface whose wettability may be altered by irradiation at the appropriate wavelength.
The ionisable moiety may be any moiety which is ionisable upon irradiation with light of a suitable wavelength and which is capable of undergoing dimerization upon ionisation. In a preferred form, the ionisable moieties contemplated by the present invention are nitrogenous heterocyclic moieties, eg. a 5-7-membered ring having 1 or 2 nitrogen atoms and preferably at least one double bond available for dimerization.
Of the nitrogenous heterocyclic moieties, a particularly preferred class of photoionisable moieties are pyrimidine-related and incorporate the substructure (I): 
wherein is an optional double bond and, where valency dictates, the trivalency of the nitrogen atom is completed by H, methyl, ethyl or propyl. In a preferred form, there is at least one double bond, more preferably a, is a double bond. In a particularly preferred form a is a double bond and b and c are single bonds.
Optionally, one or more carbon atoms may be further substituted by a substituent selected from methyl, ethyl, propyl (n- or iso-), oxo, halo (fluoro, chloro, bromo, iodo), halomethyl, hydroxy, methoxy, ethoxy, propoxy, C1-3acyloxy; amino, carboxy, carboxyethyl and carboxymethyl.
Another class of nitrogenous heterocyclic moieties contemplated are the spiropyrans, for example spiroindoline.
With respect to the pyrimidine-related moieties, the xe2x80x9cdimerizationxe2x80x9d will generally be a [2+2] cyclisation to form a cyclobutane ring. Thus, the position of the substituents and double bonds within (I) must be such that ionisation and subsequent dimerization of at least a portion of the moieties can occur upon irradiation of light with an appropriate wavelength. The placement of the substituents on (I) will affect the steric bulk of the moiety, thereby influencing the final proximity of these moieties to one another on attachment to the substrate surface and, therefore, the efficiency of the dimerization.
The person skilled in the art will also recognise that judicious selection of the substituents of (I) will influence the hydrophobicity/hydrophilicity of the photoresponsive surface.
Examples of suitably substituted pyrimidine-related moieties include thymine, uracil, cytosine, orotic acid and barbituric acid. Another class of moieties which contain the substructure (I) are the purine bases such as adenine and guanine.
The skilled person will recognise that dimerization may occur between two identical moieties or two different moieties
The steric constraints imparted by the substituents will determine whether the dimerization can be reversed. For example, where the resultant dimeric moiety is under steric strain, irradiation at a different wavelength to that which effected dimerization may cleave (i.e. reverse) the dimerization product. Thus, for example, where the moiety is thymine, where the reactive double bond is substituted by a methyl group, the resulting cyclobutane-containing dimer is more sterically strained and reversibility of the dimerization can be effected under appropriate conditions. Conversely, for example, where the moiety is uracil, the dimerization is observed as being irreversible under the same conditions which reverse the thymine dimerization. Thus by selection of appropriate substituents of (I), the dimerization, and hence the change in wettability of the surface, can be made partially or substantially fully reversible or irreversible.
The attachment of the photoionisable moieties to the substrate surface may be effected ay direct attachment to the substrate surface, or, preferably, attached via a linking group which is covalently grafted to the ionisable moiety. Where the moiety is pyrimidine-related, preferably attachment of the linking group occurs via a nitrogen atom of the photoionisable moiety.
An example of a suitable linking group for linking the photoionisable moiety to the substrate surface include alkyl chains having from 1 to about 30 carbon atoms, preferably at least 5 carbon atoms. Optionally, one or more of the carbon atoms of the linking chain (eg methylene groups) can be replaced by S, O or NH. In yet another embodiment, amide groups can be introduced into alkyl chain for additional stability (S. W. Tam-Chang, H. A. Bicbuyck, G. M. Whitesides, N. Jeon and R. G. Nuzzo, Langmuir, 11, 4371, 1995 AND J. Huang and J. C. Heminger, J. Am. Chem, Soc., 115, 3342, 1993).
Attachment of the molecules containing the photoionisable moieties to the substrate surface can occur in a number of ways, for example chemi- or electrosorption, or alternatively, by direct physical deposition onto the surface.
Where the molecule containing the ionisable moiety is chemisorbed to the substrate surface, this can occur via a moiety which is reactive with the surface of the substrate, such as a thiol group or hydroxy group. In a preferred embodiment, the reactive moiety is a thiol group. In a particular example, a thiol group can be used to chemically attach the ionisable moiety, optionally via a linking group, to a gold substrate surface. In another example, chemisorption to the surface of the fluorite or aluminum oxide surface can be achieved by a carboxylic acid group (with the pH above the pKa of the carboxylic acid group).
In another embodiment, the ionisable moiety is attached to the substrate surface via an electrosorptive process. Thus, where the substrate surface is charged, a group of the opposite charge offers the opportunity of electrostatically anchoring the moieties on the substrate For example, where the surface of the substrate is negatively charged, such as mica, electrostatic attraction between the amino group (as NH3.+at the correct pH) and the surface results in attachment. Preferably, the reactive group terminates a linking chain as described above. Electrosorptive attachment can be achieved by transfer of Langmuir-Blodgett (LB) films where a surface layer of amphiphilic molecules is compressed into a floating monolayer and transferred to a substrate by dipping.
In another embodiment, the alkyl chain lining groups terminate in a non-reactive methyl group and the molecules are deposited directly onto the substrate surface, for example by the spin cast method or by spraydrying techniques.
In order to optimize photodimerization of the moieties, a high packing density of the moieties is preferred. In addition, it is also preferable that the moieties are correctly oriented on the substrate surface. Preferably, the ionizable moieties are situated less than about 4 xc3x85 apart) preferably about 3-4 xc3x85 apart so that dimerization can occur, (Tohnai, N., Inaki, Y., Miyata, M., Yasui, N., Michizuki, E., Kai, Y., J. Photopolymer Science and Technology, 1998, 18, 59).
Self-assembled monolayers (SAMs) are one suitable means to achieve the molecular ordering of the nitrogenous heterocyclic moieties (Ulman, A. (Editor), Organic Thin Films, Directions for the Nineties, Academic Press, 1995).
Another means of achieving the desired molecular ordering is the spin-cast method.
It is also known that heat treatment, i.e. annealing, can affect the molecular orientation.
It is also within the scope of the present invention for the ionisable moieties to be fixed in proximity by molecular design of the molecules which are attached to the substrate surface, for example, more than one nitrogenous heterocyclic group may be attached to the substrate by a single linking group e.g. a branched linking group or a linking group containing a multivalent moiety such as a benzene group. This offers an advantage of providing the groups in close proximity to enhance dimerization.
One non-limiting example where the nitrogenous groups are fixed in proximity by molecular design is illustrated by 1,2-di(methylenethymyl)benzene 
Suitable substrates for use in the present invention include gold, fluorite, alumina, quartz, nickel, silica (e.g. glass), mica, zircon, TiO2 and polymeric substrates, such as fluoropolymers, where the photoionisable moiety may be directly incorporated into the monomer which is subsequently polymerised, or alternatively, where the polymer contains a reactive group for attachment of the molecule containing the photoionisable moiety. The substrate itself may be in the form of rods, plates, bars, tubes, spheres, wafers or films, and may be a smooth (peak valley roughness variations of  less than 2 nm) curved or flat surface or a physically heterogenous rough surface The skilled person will recognise which substrates will be suitable for use with particular attachment methods as referred to above.
The article having the photoresponsive surface maybe simply a film, capillary tube, cylindrical rod, small particles, hollow spheres or porous solid, (eg. membrane).
Irradiation of the photoresponsive surface is carried out at a wavelength which can be determined by the skilled person using routine methods for the particular photoionisable moiety. For the pyrimidine-related moieties, dimerization is carried out by irradiating the photoresponsive surface with light at a wavelength in the range of about 275 to about 285 nm, preferably at about 280 nm. Where steric constraints allow, reversibility of this dimerization process can be carried out at a wavelength of about 235 to about 245 nm, preferably at about 240 nm.
The present invention contemplates both photoresponsive surfaces which are chemically homogeneous and chemically heterogeneous. Chemically heterogeneous photoresponsive surfaces can be formed by attaching at least two different types of molecules to the substrate surface, either evenly, randomly or onto well defied areas of the substrate. The ionisable moieties may be varied and/or the length of the linking chain may also be varied. The photoresponsive surfaces may be attached onto the substrate surface as a monolayer or as multilayers and may be attached in defined areas of the substrate surface by masking.
An organic film surface which can switch between two states i.e., by a change in surface wettability, when subjected to an external stimulus has the potential to act as a switching device. Thus, one application of the present invention lies in the preparation and use of xe2x80x9cswitching devicesxe2x80x9d. Switching devices have applications in areas such as photoresists, non-linear optics, computer data storage and molecular recognition and self-assembly.
Another application of the described invention is in the preparation of capillary pumps which find use in X-ray applications, reversible signs and lighting arrays.
Yet another application of the invention described herein is to xe2x80x9cself-cleaningxe2x80x9d surfaces such as glass polymeric and metallic surfaces. Such applications find use in windows, mirrors, lenses, scientific and photographic equipment and in building and automotive applications.
The invention will now be described with reference to the following non-limiting Examples and Figures.